In recent years, wireless communication systems that cover wide areas such as cellular networks and WiMAX (IEEE 802.16) networks have been developed. There has also been a system in which a femto cell that is a smaller wireless communication area than a wireless communication area of a macro base station is provided as well as a wireless communication area of a base station (a macro base station) of one of the above wireless communication systems. A femto cell system is designed to eliminate a radiowave insensitive area caused when only macro base stations are provided, and to enlarge the wireless communication area. In such a femto cell system, a small-sized femto base station is placed in each subscriber's house or building.
As depicted in FIG. 1, a macro base station 200 is connected to a wireless communication carrier network 212. A wireless terminal (such as a mobile terminal) 204 in the wireless communication area 220 of the macro base station 200 can communicate with the macro base station BS through radio transmission R6. The wireless terminal 204 can communicate with other wireless terminals and application servers via the wireless communication carrier network 212. In the wireless communication system including a femto cell 222, a femto base station 202 is provided as well as the macro base station 200. In FIG. 1, the femto base station 202 is placed in the wireless communication area 220 of the macro base station 200, for example. The femto base station 202 is connected to the wireless communication carrier network 212 via a network such as the Internet 210. The wireless femto base station 202 has smaller transmission power and is less expensive than the macro base station 204. Accordingly, subscribers can easily set up the femto base station 202, and each subscriber occupy the wireless area in their houses. Since the same frequency and the same radio system are used by the macro base station 200 and the femto base station 202, the wireless terminal 204 can also communicate with the femto base station 202 through radio transmission R1.
Japanese Laid-Open Patent Publication No. 9-224283 discloses a technique for performing channel allocation by learning the characteristics of the interference wave of each radio channel between base stations and wireless terminals. Japanese Laid-Open Patent Publication No. 8-251655 discloses a technique for varying the initial value of selected order in accordance with the number of handover failures in a system that performs cellular radio channel allocation in the selected order. Japanese Laid-Open Patent Publication No. 2002-204470 discloses a technique for performing communications via a micro base station when higher-speed communications are required in a system that includes a macro base station and the micro base station that is placed in the cell of the macro base station and has higher communication speed than the macro base station.
Referring now to FIG. 2, the problems with a wireless communication system that includes a femto cell are described. In FIG. 2, a wireless terminal 204 is a terminal of an owner who has obtained and set a femto base station 202 (or a terminal that is registered as a usable terminal), and a wireless terminal 206 is another terminal (or a terminal that is not registered as a usable terminal). By setting the femto base station 202, the owner can obtain a connection with the wireless communication carrier network 212 via the femto base station 202, even when the owner is in a place where the radiowave of a macro base station 200 cannot reach. However, if the radiowave leaks from the subscriber's home, interference with the wireless communication area of the macro base station 200 is caused, and the quality of wireless communications between the macro base station 200 and the other wireless terminal 206 through radio transmission R6 is degraded. For example, in a case where the femto base station 202 is connected only to the wireless terminal 204 of the owner, the wireless terminal 206 sends a connection request to the femto base station 202 through radio transmission R2, but the connection request is denied by the femto base station 202. As a result, the unnecessary operations to make a connection request and refuse a connection are repeated between the femto base station 202 and the wireless terminal 206.